Magnetic device having multilayered free ferromagnetic layer

ABSTRACT

Magnetic multilayer structures, such as magnetic or magnetoresistive tunnel junctions (MTJs) and spin valves, having a magnetic biasing layer formed next to and magnetically coupled to the free ferromagnetic layer to achieve a desired stability against fluctuations caused by, e.g., thermal fluctuations and astray fields. Stable MTJ cells with low aspect ratios can be fabricated using CMOS processing for, e.g., high-density MRAM memory devices and other devices, using the magnetic biasing layer. Such multilayer structures can be programmed using spin transfer induced switching by driving a write current perpendicular to the layers. Each free ferromagnetic layer can include two or more layers and may be a multilayered free ferromagnetic stack that includes first and second ferromagnetic layers and a non-magnetic spacer between the first and second ferromagnetic layers.

This application is a continuation-in-part application of and thus claims the benefit of copending U.S. application Ser. No. 11/232,356 entitled “MAGNETIC DEVICE HAVING STABILIZED FREE FERROMAGNETIC LAYER” and filed Sep. 20, 2005, now U.S. Pat. No. 7,777,261 the entire disclosure of which is incorporated by reference as part of the specification of this application.

BACKGROUND

This application relates to magnetic materials and structures having at least one free ferromagnetic layer.

Various magnetic materials use multilayer structures which have at least one ferromagnetic layer configured as a “free” layer whose magnetic direction can be changed by an external magnetic field or a control current. Magnetic memory devices may be constructed using such multilayer structures where information is stored based on the magnetic direction of the free layer.

One example for such a multilayer structure is a magnetic or magnetoresistive tunnel junction (MTJ) which includes at least three layers: two ferromagnetic layers and a thin layer of a non-magnetic insulator as a barrier layer between the two ferromagnetic layers. The insulator for the middle barrier layer is not electrically conducting and hence functions as a barrier between the two ferromagnetic layers. However, when the thickness of the insulator is sufficiently thin, e.g., a few nanometers or less, electrons in the two ferromagnetic layers can “penetrate” through the thin layer of the insulator due to a tunneling effect under a bias voltage applied to the two ferromagnetic layers across the barrier layer. Notably, the resistance to the electrical current across the MTJ structure varies with the relative direction of the magnetizations in the two ferromagnetic layers. When the magnetizations of the two ferromagnetic layers are parallel to each other, the resistance across the MTJ structure is at a minimum value R_(P). When the magnetizations of the two ferromagnetic layers are anti-parallel with each other, the resistance across the MTJ is at a maximum value R_(AP). The magnitude of this effect is commonly characterized by the tunneling magnetoresistance (TMR) defined as (R_(AP)−R_(P))/R_(P).

The relationship between the resistance to the current flowing across the MTJ and the relative magnetic direction between the two ferromagnetic layers in the TMR effect may be used for nonvolatile magnetic memory devices to store information in the magnetic state of the MTJ. Magnetic random access memory (MRAM) devices based on the TMR effect, for example, may be an alternative of and compete with electronic RAM devices. In such devices, one ferromagnetic layer is configured to have a fixed magnetic direction and the other ferromagnetic layer is a “free” layer whose magnetic direction can be changed to be either parallel or opposite to the fixed direction. Information is stored based on the relative magnetic direction of the two ferromagnetic layers on two sides of the barrier of the MTJ. For example, binary bits “1” and “0” may be recorded as the parallel and anti-parallel orientations of the two ferromagnetic layers in the MTJ. Recording or writing a bit in the MTJ can be achieved by switching the magnetization direction of the free layer, e.g., by a writing magnetic field generated by supplying currents to write lines disposed in a cross stripe shape, by a current flowing across the MTJ based on the spin transfer effect, or by other means. In the spin-transfer switching, the current required for changing the magnetization of the free layer can be small (e.g., 0.1 mA or lower) and can be significantly less than the current used for the field switching. Therefore, the spin-transfer switching in a MTJ can be used to significantly reduce the power consumption of the cell.

The need for high storage capacity in a magnetic memory device based on MTJ cells requires each MTJ cell to be small in order to increase the number of MTJ cells in a given wafer area. As the size of a MTJ cell reduces, the magnetization direction of the MTJ in each cell can become increasingly sensitive to various factors such as thermal fluctuations, external field disturbances or superparamagnetism. This is in part because the magnetic energy due to the coercivity of the MTJ for storing and maintaining a digital bit reduces with the size of the MTJ cell. When the magnetic energy for storing and maintaining the digital bit is reduced with the cell size below a critical level which usually is multiple times the energy of a source of disturbance, the energy of the disturbance may be sufficient to alter the magnetic state of the MTJ cell and thus change the stored bit. Therefore, the magnetization direction of the MTJ in a sufficiently small cell may unexpectedly change because of any one or a combination of these and other factors and thus alter or erase the stored information in the MTJ. The disturbance may be caused by various factors, such as the thermal energy of the thermal fluctuation around the cell or the energy due to interaction between the MTJ cell and an astray magnetic field present at the cell.

Therefore, it is desirable to increase the coercivity of the free ferromagnetic layer in MTJs and other multilayer structures and thus to stabilize the magnetic direction of the free ferromagnetic layer against various disturbances.

SUMMARY

This application describes, among other features, magnetic or magnetoresistive tunnel junctions (MTJs) and other magnetic multilayer structures that use a magnetic biasing layer to increase coercivity of the free layer for improved magnetic stability against thermal fluctuations and astray fields. Such MTJs and other magnetic multilayer structures may be used to construct magnetic memory cells in highly integrated circuits such as high-density integrated MRAM chips based on CMOS processing where each cell is small with a low aspect ratio.

In one example, a device is described to include a free ferromagnetic layer having a magnetization direction that is changeable between a first direction and a second opposite direction. A magnetic biasing layer is also formed in this device to be in contact with and magnetically coupled to the free ferromagnetic layer to increase coercivity of the free ferromagnetic layer and to allow the magnetization direction of the free ferromagnetic layer to be changeable between the first direction and the second substantially opposite direction. This device also includes a fixed ferromagnetic layer having a magnetization direction fixed along substantially the first direction and an insulator barrier layer formed between the free and fixed ferromagnetic layers to effectuate tunneling of electrons between the free and fixed ferromagnetic layers under a bias voltage applied between the free and fixed ferromagnetic layers and across the insulator barrier layer. The free ferromagnetic layer is located between the magnetic biasing layer and the insulator barrier layer.

In another example, a method is described to include the following. A magnetic tunnel junction is provided to comprise a free ferromagnetic layer having a changeable magnetization direction, a fixed ferromagnetic layer having a fixed magnetization direction, and an insulator barrier layer formed between the free and fixed ferromagnetic layers to effectuate tunneling of electrons between the free and fixed ferromagnetic layers under a bias voltage applied between the free and fixed ferromagnetic layers and across the insulator barrier layer. A magnetic biasing layer is also provided to be in contact with and magnetically coupled to the free ferromagnetic layer to increase coercivity of the free ferromagnetic layer while allowing a magnetization direction of the free ferromagnetic layer to be changeable. The free ferromagnetic layer is located between the magnetic biasing layer and the insulator barrier layer.

In yet another example, this application describes a device that includes a free ferromagnetic layer having a magnetization direction that is changeable, and a magnetic biasing layer formed to be in contact with and magnetically coupled to the free ferromagnetic layer to increase coercivity of the free ferromagnetic layer without pinning a magnetization direction of the free ferromagnetic layer. The magnetic biasing layer has a layer thickness t, an anisotropy constant K and an interface exchange coupling constant J to satisfy K·t<J. The MTJ also includes a pinned ferromagnetic layer having a magnetization direction fixed along a predetermined direction and an antiferromagnetic pinning layer in contact with and magnetically coupled to the pinned ferromagnetic layer to cause the magnetization direction of the pinned ferromagnetic layer to be fixed along the predetermined direction. The antiferromagnetic pinning layer has a layer thickness t′, an anisotropy constant K′ and an interface exchange coupling constant J′ to satisfy K′·t′>J′. The device further includes a middle layer formed between the free and pinned ferromagnetic layers. The free ferromagnetic layer is located between the magnetic biasing layer and the middle layer. In one implementation, the middle layer may be an insulator barrier layer formed between the free and pinned ferromagnetic layers to effectuate tunneling of electrons between the free and pinned ferromagnetic layers under a bias voltage across the insulator barrier layer. In another implementation, the middle layer may be a non-magnetic metal layer.

This application further describes MTJ structures with at least a multilayered free ferromagnetic stack. In one example, a device includes a substrate and a magnetic cell formed on the substrate. The magnetic cell includes a multilayered free ferromagnetic stack having a net magnetization direction that is changeable between a first direction and a second substantially opposite direction. The multilayered free ferromagnetic stack includes first and second ferromagnetic layers and a non-magnetic spacer between the first and second ferromagnetic layers. A fixed ferromagnetic layer is included in the magnetic cell to have a fixed magnetization direction. The magnetic cell also includes an insulator barrier layer formed between the multilayered free ferromagnetic stack and fixed ferromagnetic layer to effectuate tunneling of electrons between the multilayered free ferromagnetic stack and fixed ferromagnetic layer under a bias voltage applied between the multilayered free ferromagnetic stack and fixed ferromagnetic layer and across the insulator barrier layer. In another example, the above device can include a magnetic biasing layer in contact with and magnetically coupled to the multilayered free ferromagnetic stack to increase coercivity of the multilayered free ferromagnetic stack and to allow the magnetization direction of the multilayered free ferromagnetic stack to be changeable between the first direction and the second substantially opposite direction.

These and other implementations, their variations and modifications are described in greater detail in the attached drawings, the detailed description, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows one example of a conventional MTJ cell structure without a magnetic biasing layer.

FIGS. 1B and 1C show a MTJ array on a chip and an example of a structure of a CMOS-based chip layout, where each MTJ cell may use a MTJ with a magnetic biasing layer.

FIG. 2 shows an example of a MTJ with a magnetic biasing layer to increase coercivity of a coupled free layer.

FIGS. 3A and 3B show magnetic properties of exemplary magnetic biasing layers and the coupled free layers based on the MTJ design in FIG. 2.

FIGS. 4 and 5 show two examples of MTJs using magnetic biasing layers.

FIGS. 6A and 6B show two alternative implementations of the cell design in FIG. 2 to reduce the dimension of the free layer and to reduce the spin-transfer based switching current.

FIGS. 7A and 7B show two alternative implementations of the cell design in FIG. 5 to reduce the dimension of the free layers and to reduce the spin-transfer based switching current.

FIG. 8 shows an example of an MTJ structure where a free layer is a sandwich multilayer structure having first and second ferromagnetic layers and a non-magnetic spacer layer between the two ferromagnetic layers.

FIG. 9 illustrates the offset in the magnetization directions between the weakly pinned free magnetic layer and the pinned layer for one implementation of the MTJ structure in FIG. 8.

DETAILED DESCRIPTION

The techniques of using a magnetic biasing layer to stabilize a free ferromagnetic layer described in this application may be applied to a variety of magnetic multilayer structures. In various implementations, the magnetic direction of the free ferromagnetic layer may be switched by using a spin transfer effect. An MTJ is only one example of such structures. Another example of such a multilayer structure having a free ferromagnetic layer is a spin valve structure which can also be used in magnetic memory devices and other magnetic devices. The spin valve can include two ferromagnetic layers and a thin layer of a non-magnetic metal layer as a spacer layer between the two ferromagnetic layers. Similar to MTJs, one ferromagnetic layer is fixed and the other is a free layer. The free layer in the spin valve is subject to the similar stability issues as in MTJs. Thus, the biasing layer may also be implemented in the spin valve. The following examples use MTJs as examples to illustrate the designs, examples, and operations of various biasing layers.

FIG. 1A illustrates an example of a MTJ 100 formed on a substrate 101 such as a Si substrate. The MTJ 100 is constructed on one or more seed layers 102 directly formed on the substrate 101. On the seed layers 102, an antiferromagnetic (AFM) layer 113 is first formed and then a first ferromagnetic layer 111 is formed on top of the AFM layer 113. After the post annealing, the ferromagnetic layer 111 later is pinned with a fixed magnetization. In some implementations, this fixed magnetization may be parallel to the substrate 101 (i.e., the substrate surface). On top of the first ferromagnetic layer 111 is a thin insulator barrier layer 130 such as a metal oxide layer. In the MTJ 100, a second ferromagnetic layer 112 is formed directly on top of the barrier layer 130. In addition, at least one capping layer 114 is formed on top of the second ferromagnetic layer 112 to protect the MTJ.

The magnetization of the ferromagnetic layer 112 is not pinned and can be freely changed to be parallel to or anti-parallel to the fixed magnetization of the pinned layer 111 under a control of either an external control magnetic field or a driving current perpendicularly flowing through the MTJ. For this reason, the layer 112 is a free layer (FL). A magnetic field in the field operating range, or an applied current across the junction in the current operating range, can force the magnetization of the free layer 112 to be substantially parallel to or substantially opposite to the fixed magnetization of the pinned layer 111. Many magnetic systems have competing energy contributions that prevent a perfect parallel or antiparallel alignment of the magnetic domains or nanomagnets in each ferromagnetic layer. In MTJs, the dominant contribution to the energy state of the nanomagnets within the free layer 112 tends to force the nanomagnets into the parallel or antiparallel alignment, thus producing a substantial parallel or antiparallel alignment.

FIG. 1B shows an example of a magnetic memory chip device where a two-dimensional array of MTJ cells based on the MTJ design in FIG. 1A or other MTJ designs are monolithically formed on the substrate 101. A Cartesian coordinate (x, y, z) is used to illustrate different dimensions of the chip. Rectangular blocks in FIG. 1B are used to represent relative positions of MTJ cells and dimensions of each MTJ cell of the memory chip. In an actual device, each cell may be elliptically shaped and elongated along the x direction. FIG. 1C illustrates one exemplary layout of various structures including MTJs of the memory chip in FIG. 1B where each MTJ is fabricated on a metal via plug on the substrate. Each MTJ cell is illustrated to have a length L1 along the x direction and a length L2 along the y direction, wherein both x and y directions are parallel to the plane of the substrate. The aspect ratio of each MTJ cell in the x and y directions is A=L1/L2. In order to increase the storage areal density of the memory chip, both L1 and L2 are reduced to increase the number of MTJ cells in a given area in a memory device.

One known technique to increase the thermal stability of the MTJ cells uses the shape anisotropy of the magnetic recording layer of the magnetic cell to spatially favor a particular magnetization direction. In some cases, large shape anisotropy may be used to compensate for the insufficient amount of intrinsic crystalline anisotropy which may be, e.g., from several to tens of Oersted in terms of an anisotropy field. According to a static-magnetic model, the switching field for an elliptically shaped MTJ cell can be expressed for the films having in-plane dominant anisotropy as H _(Keff) =H _(Kins) +H _(Kshape) where H_(Kins) represents the anisotropy field due to crystalline anisotropy and H_(Kshape) represents the anisotropy field due to the shape anisotropy. Notably, H_(Kshape) is proportional to A·t_(F)/L₁ where A is aspect ratio of the MTJ in a plane parallel to the MTJ layers, L₁ is the length along the long axis of the magnetic cell and t_(F) is the thickness of the free layer. Evidently, A should be larger than one, in order to maintain a sufficiently large H_(Kshape) and thus a large effective anisotropy H_(Keff) to meet the thermal stability requirements imposed on the cells. The large anisotropy corresponds to a large thermal activation factor of (K_(u)V/k_(B)T), where K_(u) is the uniaxial anisotropy energy and V is the volume of the free layer.

However, the scaling of the magnetic cell embedded into CMOS manufacturing process may impose limitations to the size, geometry and aspect ratio (A) of the cell. For example, the 130-nm-node CMOS technology can limit the upper limit of the aspect ratio A of the MTJ cells to about 1.77 if the overlap rule is ignored and to around 1 if the overlap rule is taken into account for designing a via size of 0.23 μm with an overlap of 0.055 μm per side. When the more advanced technology node of 90 nm is used, the aspect ratio A of the MTJ cells is actually reduced to 1 from 1.67 for a via size of 0.15 μm with an overlap of 0.03 μm per side. Therefore, due to the CMOS fabrication limitations to the aspect ratio A of each cell, it may be difficult to achieve both a large aspect ratio A and a high cell density at the same time. As such, the approach to stabilizing a MTJ cell based on the shape anisotropy is difficult to implement in memory devices with high areal cell densities. In addition, cells with asymmetric shapes may increase the process complexity during fabrication and the uniformity of the cells may be difficult to control.

This application describes techniques and MTJ cell designs that produce a large anisotropy field or coercivity of the free layer in the MTJ to improve the stability of the MTJ cell against disturbances such as thermal fluctuations and astray fields without requiring the cell shape to be asymmetric. A magnetic cell with a high thermal stability can be achieved with a low aspect ratio due to the large anisotropy field or coercivity of the free layer in the MTJ cell provided by a biasing layer. As a result, standard CMOS fabrication processes can be compatible with fabrication processes of such MTJ cells in small dimensions to construct highly integrated MTJ cell arrays used in MRAM and other devices. In various implementations, the MTJ cells with free layers of high coercivity may be designed to operate based on the spin-transfer induced magnetization switching for recording data. Such MTJ cells may be used to achieve low power consumption and fast writing and readout.

FIG. 2 shows one example of a MTJ cell design 200 which has a free ferromagnetic layer 202 of high coercivity suitable for achieving a desired level of thermal stability at a low aspect ratio. The basic layers for the MTJ in FIG. 2 are similar to the design in FIG. 1. The free layer 202 is different from the free layer 112 in FIG. 1 in that a magnetic biasing layer 201 is formed in contact with and is magnetically coupled to the free layer 202 to increase the coercivity of the free layer 202. The magnetic coupling between the magnetic biasing layer 201 and the free layer 202 is set at a level to allow the magnetization direction of the free layer 202 to be changeable or switched between two opposite directions by, e.g., using a driving current through the MTJ based on the spin-transfer switching. The pinned layer 111 has a fixed magnetization direction which may be along either the first or second direction. The insulator barrier layer 130 is formed between the free and pinned layers 202 and 111 to effectuate tunneling of electrons between the free and pinned layers 202 and 111 under a bias voltage applied between the free and pinned layers 202 and 111 and across the insulator barrier layer 130. The metal for forming the insulator barrier layer 130 may be, for example, aluminum (Al), hafnium (Hf), zirconium (Zr), tantalum (Ta) and magnesium (Mg). In addition, various nitride layers based on different metals may be used to implement the insulator barrier layer 130. Some examples are an aluminum nitride (e.g., AlN), a Ti nitride (e.g., TiN), an AlTi nitride (e.g., TiAlN) and a magnesium nitride. Each of the layers 111, 202 and 114 may have a multilayer structure to include two or more sublayers. The magnetic biasing layer 201 may be antiferromagnetic or ferrimagnetic.

In this design, the ferromagnetic layer 111 is in contact with the AFM layer 113 and is magnetically coupled to the AFM layer 113. The ferromagnetic layer 111 is not “free” and cannot be switched because its magnetization direction is fixed by the AFM layer 113. The AFM layer 113 is specifically designed to pin the magnetization direction of the ferromagnetic layer 111. In this context, the AFM layer 113 may be characterized by three parameters: its layer thickness t_(AF), its anisotropy constant K_(AF) and its interface exchange coupling constant J_(int) with the ferromagnetic layer 111. When these parameters for the AFM layer 113 meet the condition of K _(AF) ·t _(AF) >J _(int), the magnetic anisotropy of the AFM layer 113 dominates and the AFM layer 113 magnetically controls the anisotropy of the layer 111 via the magnetic coupling between the layers 113 and 111. Under this condition, the magnetization direction of the ferromagnetic layer 111 is fixed by the unidirectional anisotropy of the AFM layer 113. This pinning condition may be achieved by, e.g., using a large AFM layer thickness t_(AF), an AFM material with a large anisotropy constant K_(AF), or both large t_(AF) and large K_(AF). The pinning condition can be achieved with an AFM material which has a large AFM layer thickness t_(AF) but a relatively small K_(AF).

The magnetic biasing layer 201 is designed to be magnetically different from the AFM layer 113 for pinning the layer 111 and to provide a different function from the AFM layer 113. Although the layer 201 and the free layer 202 are magnetically coupled to each other, the free layer 202 is still “free” and its magnetization direction can be changed by the driving current based on the spin-transfer switching. As such, the biasing layer 201 is designed to meet the following condition: K _(AF) ·t _(AF) <J _(int). When the thickness of the magnetic biasing layer 201 is set to be small, the exchange bias field can be negligibly small but the coercivity increases with the AFM layer thickness due to the increase of the total anisotropy energy in the free layer 202. Hence, the magnetic biasing layer 201 is designed to provide a large anisotropy field for the free layer 202. In various implementations, the AFM material for the magnetic biasing layer 201 is selected to have a blocking temperature higher than the operating temperatures of the MTJ cell, a large interface exchange coupling constant J_(int), and an appropriately large anisotropy constant K_(AF). For antiferromagnetic materials, the Neel temperature is the blocking material. For a ferrimagnetic material, its Curie temperature is the blocking temperature. In many applications, the thickness t_(AF) of the magnetic biasing layer 201 may be set at a fixed value or changeable. The other two parameters K_(AF) and J_(int) are, therefore, adjusted and selected to achieve the condition of K_(AF) ·t _(AF)<J_(int) so that the anisotropic field or coercivity Hc in the free layer 202 is sufficient to match the requirement of the thermal stability for the magnetic cell design.

For fixed values of J_(int) and K_(AF), the critical AFM thickness is t_(AFcritical)=J_(int)/K_(AF) and is used as an indicator of set-on of the exchange bias field Hex between the two operating regimes. For two widely used AFM materials IrMn and PtMn, the estimated values for t_(AFcritical) are 40 and 80 Å, respectively with J_(int)=0.04(IrMn) and 0.08(PtMn) erg/cm² and K_(AF)=1×10⁺⁵ erg/cm³. In actual device implementations, the values may vary from the above estimates due to various complexities in fabrication processes.

The magnetic biasing layer 201 is designed to be within the regime of K_(AF)·t_(AF)<J_(int) to deliver a sufficiently large coercivity for the adjacent free layer 202. As an example, for a magnetic cell design with K_(u)V/k_(B)T=55 which may be required for data retention for 10 years, the corresponding coercivity in the free layer is about 100 Oe when an IrMn AFM layer is used as the magnetic biasing layer for Area=0.02 μm², t_(F)=25 Å, Ms=1050 emu/cc, and A=1:1. However, the existing experimental data suggest that most noticeable Hc will be within tens of Oersted and that Hc usually also increases with the exchange bias field Hex. In such a circumstance, the use of the magnetic biasing layer with the free layer to enhance the anisotropy of the latter may necessitate an AFM stack structure and process engineering to suppress the Hex but enhance coercivity within the regime of K_(AF)·t_(AF)<J_(int).

FIG. 3A shows a mode of Hc enhancement in the regime of K_(AF)·t_(AF)<J_(int) where K_(AF)=1×10⁺⁵ erg/cm³, t_(F)=25 Å, and Ms=1050 emu/cc. FIG. 3B shows an anisotropic energy from an AFM magnetic biasing layer (IrMn) requiring K_(AF)·t_(AF) to be 0.01 erg/cm² or more for producing a large coercivity Hc in the free layer.

The magnetic biasing layer 201 in FIG. 2 may be implemented in various configurations. For example, the magnetic biasing layer 201 may be made of a single AFM layer with an offset composition via process condition control such that the Mn content ranges from 50 to 95 at. % for IrMn, and from 30 to 80 at. % for PtMn. The layer thickness may be from 10 to 200 Å.

As another example, the magnetic biasing layer 201 may be a bilayer of two AFM sublayers respectively made from two AFM materials AFM1 and AFM1, or a stack of the bilayers of AFM1 and AFM2. Each AFM sublayer may have a thickness of 10-200 Å and may be formed from IrMn or PtMn. For example, the sublayer AFM1 may have the Mn content within 75-85 at. % for IrMn or 45-55 at. % for PtMn. The layer thickness may be 20-40 Å for IrMn and 40-80 Å for PtMn. The sublayer AFM2 may have the Mn content within 50-85 or 85-95 at. % for IrMn, and 5-45 or 55-80 at. % for PtMn with an adjustable layer thickness.

The magnetic biasing layer 201 may also be a trilayer of three layers where a non-magnetic spacer placed between two AFM layers AFM1 and AFM2 (i.e., AFM1/spacer/AFM2), or a multi-layer stack of two or more bilayers AFM1/spacer (Spacer=Ru, Ta, Cu, NiFeCr, 2-20 Å).

In these and other examples, the AFM1 and AFM2 layers may have similar or different compositions and structures. In the magnetic biasing layer with two or more sublayers, the AFM sublayer which interfaces with the free layer is designed to have a thickness less than the AFM critical layer thickness. This provides further control of exchange bias field Hex while enhancing the coercivity Hc as the total magnetic biasing layer grows.

Magnetron sputtering may be used to fabricate the magnetic biasing layers. Alternatively, an ion beam assisted deposition process using IBD can be used to modify the layer structure and content to enhance Hc but reduce Hex. In addition, ion irradiation of a magnetic biasing layer in a field may be used to enhance Hc while decreasing Hex.

Various AFM materials may be used as to construct one or more sublayers of the entire magnetic biasing layer 201. For example, any or a combination of any two or more the following AFM materials may be used: (1) metallic AFM materials: IrMn, PtMn, PtPdMn, NiMn, FeMn(Rh), CrMn, FeNiMn, CoMn, RhMn, CrMn(Pt, Cu, Rh, Pd, Ir, Ni, Co, Ti), and CrAl; (2) oxide AFM materials: Ni(Fe)O, Fe(Co)O, Co(Fe)O, NiFe(Co)O, CrO; and (3) FeS. In addition, each AFM material for the magnetic biasing layer 201 may also be replaced by one or a combination of two or more ferrimagnetic materials like TbCo, DyCo, TbFe, TbFeCo, CoFeO, FeO, and (Mn, Zn)FeO. In the above composition expressions, the element in ( ) represents a content-poor element. For example, in Fe(Co)O, the content of Co is less than Fe.

The via processing in CMOS design and manufacturing imposes overlap rules and thus limits the aspect ratio and size of a magnetic cell. MTJ cells using the magnetic biasing layers improve the thermal stability of the free layer and allow for stable MTJ cells fabricated within the limitations of the CMOS processing. The free layer may be designed with a small thickness and a small magnetic moment to reduce the spin-transfer switching current density needed for switching the magnetization direction of the free layer based on the spin transfer effect. The presence of the magnetic biasing layer allows for such a thin free layer to be achieved with enhanced coercivity and desired thermal stability.

Once the size and aspect ratio of MTJ cells are reduced to below a dimension of 200 nm or under 100 nm, such small MTJ cells tend to be thermally unstable and subject to the edge effect of the magnetic domain structure and astray field influence. The edge effect tends to reorient the spin states by causing spin curling up or creating spin vortex states. The reorientation of spin can degrade the magnetic performance of magnetic cells and increase the data error rate in information storage. The use of the magnetic biasing layer may be used to address these issues in various implementations so that the spins align along the easy axis of the magnetic biasing layer due to anisotropic energy interaction between the magnetic biasing layer and free layer, improving the magnetic performance of the magnetic cells.

Notably, the enhanced coercivity in the free layer due to the magnetic interaction with the magnetic biasing layer allows the MTJ cell to achieve a desired level of stability against thermal fluctuations and astray magnetic fields without relying on the shape anisotropy of the cell. As such, if the degree of the cell shape anisotropy is limited, such as when the cells are fabricated with CMOS processing to have a dimension around 100 nm, the use of the magnetic biasing layer allows the MTJ cells to be designed and fabricated to comply with the low aspect ratios imposed by the CMOS processing techniques. In this context, the geometry or aspect ratio of magnetic cells is no longer a limiting factor to the MTJ cells. Therefore, the use of the magnetic biasing layer can facilitate the cell design and layout. In addition, the coercivity of the free layer can be set to a desired amount by tuning the anisotropy of the magnetic biasing layer via structure and process control. This tuning can be achieved with relative ease and with improved uniformity and process margin in comparison to controlling of the aspect ratio of each cell in CMOS processing.

Various examples for layers other than the magnetic biasing layer in the design in FIG. 2 are described below.

1. Free Layer

The free layers (FL) are Co, Fe, Ni or their alloys with crystalline structure or with amorphous states modified by boron or other amorphous forming elements addition at different composition (0-30 at. %). The saturation magnetization of the free layer can be adjusted between 400-1500 emu/cm³ by varying the composition of amorphous forming elements. The layer thickness may be controlled so that the output signal (while optimizing current induced switching) remains at an acceptable level.

The free layer could be a single layer or a multiple layer configuration. For a single layer case, a ferromagnetic or ferrimagnetic material can be used. The individual layers of the multiple layer configurations could be either a combination of magnetic materials, which are either ferromagnetic or ferrimagnetic, or a combination of magnetic and non-magnetic layers (such as synthetic anti-ferromagnetic where two ferromagnetic layers are separated by a non-magnetic spacer). The spacer layer used in this synthetic structure also provides advantage of a diffusion stop layer against the possible diffusion of Mn element used in an antiferromagnetic layer into a barrier layer. The ferromagnetic layers could be Co, CoFe(5-40%), CoFe(5-40%)B(5-30%), CoFe(5-40%)Ta(5-30%), NiFe(˜20%), CoPt(5-40%), CoPd(5-40%), FePt(5-40%), CO₂Mn(Al, Si) or CO₂(Cr,Fe)(Al, Si). Ferrimagnetic layers could be CoGd(15-35%) or FeGd(10-40%). The non-magnetic spacer could be Ru, Re or Cu. All compositions are in atomic percent.

2. Pin Layer

The pin layers (PL) are Co, Fe, Ni or their alloys with crystalline structure or with amorphous states modified by boron or other amorphous forming elements addition at different composition (0-30 at. %). The pin layer could be a single layer or a multiple layer configuration. For a single layer case, a ferromagnetic or ferrimagnetic material can be used. The individual layers of the multiple layer configurations could be either a combination of magnetic materials, which are either ferromagnetic or ferrimagnetic, or a combination of magnetic and non-magnetic layers (such as synthetic anti-ferromagnetic where two ferromagnetic layers are separated by a non-magnetic spacer). The ferromagnetic layers could be Co, CoFe(5-40%), CoFe(5-40%)B(5-30%) CoFe(5-40%)Ta(5-30%), NiFe(˜20%), CoPt(5-40%), CoPd(5-40%), FePt(5-40%), CO₂Mn(Al, Si) or CO₂(Cr,Fe) (Al, Si). Ferrimagnetic layers could be CoGd(15-35%) or FeGd(10-40%). The non-magnetic spacer could be Ru, Re or Cu. All compositions are in atomic percent.

3. Barrier Layer

The tunneling barrier layer could be either single layers of AlO(40-70%), MgO(30-60%), AlO(40-70%)N(2-30%), AlN(30-60%) and Al(Zr, Hf, Ti, Ta)O or a multilayer of the above films with crystalline structure or with amorphous states. The barrier layers with thickness between 5 Å and 40 Å are processed by depositing original metal starting material and then oxidizing the deposited films using natural oxidation and/or plasma oxidation, or by rf sputtering original oxide starting material so that there is tunneling current across the barrier. The resistance-area product range of the barrier is between 10 and 100 Ω-μm². The structure of the interfaces between the barrier and free layer as well as the barrier and the pinned layer are optimized to get maximum spin polarization of electrons (polarization>40%) as well as maximum tunneling magneto-resistance (TMR) values (e.g., TMR>20%).

4. Spacer Layer

In a spin valve cell, the barrier layer 130 described above for MTJ cells is replaced by a non-magnetic metal spacer layer. Examples for the spacer material include Cu, Ag, Pt, Ru, Re, Rh, Ta, Ti, combinations of two or more these metals, or alloys of these metals. The non-magnetic spacer layer may be made of one or more of the above metals in combination with a nano-oxide (NOL) layer or current confinement layer insertion. In some implementations, the non-magnetic spacer may be formed by first depositing original metal starting material and then oxidizing the deposited films using natural oxidation and/or plasma oxidation, or by rf sputtering an original oxide starting material. The starting metal material may use the materials similar to pin or free layer material such as magnetic material CoFe, CoFeB, and non magnetic material Al, Ta, Ru, and Ti. The current confinement layer could be Cu/CoFe, FeSi, Al, Ta, Ru or Ti/NOL/Cu for instance.

The MTJ cell with a magnetic biasing layer in FIG. 2 uses a “bottom MTJ” configuration where the pinned ferromagnetic layer 111 is located between the barrier layer 130 and the substrate 101. Alternatively, the free layer 202 and its magnetic biasing layer 201 may be placed under the barrier layer 130 and above the substrate 101 in a “top MTJ” configuration. FIG. 4 shows one example 400 of such an MTJ where the magnetic biasing layer 201 is directly grown on top of the seed layer(s) 102.

More complex structures may be constructed based on the above MTJs with magnetic biasing layers. FIG. 5 shows a dual MTJ structure 500 where two MTJs 510 and 520 are stacked over each other and share a common magnetic biasing layer 501. In this example, the first MTJ 510 has a pinning AFM layer 113A directly formed on the top of the seed layers 102 and other layers of the MTJ 510, the pinned layer 111A, the barrier layer 130A, the free layer 511 and the magnetic biasing layer 501, are sequentially placed above the AFM layer 113A. The magnetic biasing layer 501 for the MTJ 510 is also the magnetic biasing layer for the MTJ 520 which includes a free layer 521, a barrier layer 130B, a pinned layer 111B and a pinning AFM layer 113B that are sequentially placed on top of the magnetic biasing layer 501. One or more capping layers 114 are then formed on the top of the MTJ 520.

In the above magnetic cell designs, the current used to switch the magnetization direction of the free layer based on the spin transfer can be reduced by reducing the lateral dimension of the free layer. Reduction of the switching current can reduce the power consumption of the memory cell and mitigate thermal dissipation issues in highly integrated memory cell designs. In addition to having the biasing layer to enhance the thermal stability of the thin free layer, the cell geometry may be designed to achieve a lower spin transfer switching current density.

FIGS. 6A and 6B show two cell profile designs for implementing the cell design in FIG. 2 with a reduced dimension of the free layer. In FIG. 6A, the side profile of the multilayer stack 601 for each cell is sloped or tapered to reduce the lateral dimension of the free layer 202. In FIG. 6B, the multilayer stack 602 for each cell is shaped as a reverse T shape to have a mesa top which includes the free layer 202 and the magnetic biasing layer 201 and a bottom with the barrier layer 130, the pinned layer 111, and the AFM layer 113, the seed layer 102. The dimension of the mesa top is less than the dimension of the bottom to reduce the lateral dimension of the free layer in the mesa top. Similarly, FIGS. 7A and 7B show two cell profile designs for implementing the cell design in FIG. 5 with a reduced dimension of the free layer.

The above sloped sidewall profile in FIGS. 6A and 7A may be obtained by a sloped masking layer during processing, and the reverse T shape in FIGS. 6B and 7B may be obtained by using a sidewall spacer process.

The above illustrated examples for a MTJ structure may also be replaced by a spin valve structure by replacing the MTJ barrier insulator layer with a non-magnetic metal spacer layer. In FIG. 5, for example, the dual structure shown can be a dual spin filter which stacks a spin valve and a MTJ over each other by replacing one of the two barriers with a nonmagnetic spacer layer.

An MRAM memory devices based on the above MTJ cells with magnetic biasing layers can be designed to use the spin-transfer switching for recording bits in the cells. Circuit elements for making row selection and column selection, including word lines and bit lines are included in the MRAM memory device. A write current source for supplying the polarized current for recording data and a read current source for supplying a sensing current for readout are also provided in the memory device. The spin transfer switching in such a memory device can be implemented in various configurations. Current knowledge of spin transfer is described in detail in J. C. Slonczewski, “Current-driven Excitation of Magnetic Multilayers,” Journal of Magnetism and Magnetic Materials, vol. 159, p. L1-L5 (1996); L. Berger, “Emission of Spin Waves by a Magnetic Multilayer Traversed by a Current,” Phys. Rev. B, Vol. 54, p. 9353 (1996), and in F. J. Albert, J. A. Katine and R. A. Burhman, “Spin-polarized Current Switching of a Co Thin Film Nanomagnet,” Appl. Phys. Lett., vol. 77, No. 23, p. 3809-3811 (2000). The spin transfer can be, however, implemented in other configurations beyond the techniques described in the above referenced publications.

Based on the above referenced publications, the spin-transfer effect arises from the spin-dependent electron transport properties of ferromagnetic-normal metal multilayers. When a spin-polarized current traverses a magnetic multilayer structure in a direction perpendicular to the layers, the spin angular momentum of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and normal-metal layers. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, a spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high (approximately 10⁷-10⁸ A/cm²), and if the dimensions of the multilayer are small (approximately less than two hundred nanometers) so that effects of the helical magnetic field produced by the perpendicular current are not important. In addition, to reduce the switching current for changing the magnetization direction of the free layer, the ferromagnetic layer should be sufficiently thin, for instance, preferably less than approximately ten nanometers for Co in some implementations. A thin free layer, in absence of the magnetic biasing layer, has less coercivity and is less stable than a thicker free layer which requires a greater switching current. The use of the magnetic biasing layer allows for the free layer to be thin to achieve a small switching current while still maintaining a sufficient level of coercivity in the free layer against thermal fluctuations and astray magnetic fields.

As described above, a free ferromagnetic layer in the examples of various magnetic structures can include two or more layers. Such a multilayered free ferromagnetic layer can be implemented in various configurations. For example a multilayered free ferromagnetic layer can include a combination of magnetic materials and each of the magnetic materials can be either ferromagnetic or ferrimagnetic. A ferromagnetic material in such a combination for the free ferromagnetic layer can include Co, an alloy comprising Co, Fe, an alloy comprising Fe, Ni, or an alloy comprising Ni. Hence, as a specific example, the ferromagnetic material in such a combination for the free ferromagnetic layer can include two or more of Co, an alloy comprising Co, Fe, an alloy comprising Fe, Ni, and an alloy comprising Ni. Some examples of ferromagnetic alloys suitable for a combination for the free ferromagnetic layer include CoFe, CoFeB, CoFeTa, NiFe, CoPt, CoPd, FePt, CO₂Mn(Al, Si) and CO₂(Cr,Fe) (Al, Si). A ferrimagnetic material for the free ferromagnetic layer can be, for example, CoGd or FeGd.

Notably, a multilayered free ferromagnetic layer can include a combination of magnetic and non-magnetic layers. One specific example is a sandwich multilayer structure having two ferromagnetic layers and a non-magnetic spacer layer between the two ferromagnetic layers.

FIG. 8 shows an example of an MTJ structure 800 where a multilayered free ferromagnetic stack 810 is a sandwich multilayer structure having first and second ferromagnetic layers 811 and 812 and a non-magnetic spacer layer 813 between the two ferromagnetic layers 811 and 812. The non-magnetic spacer layer 813 is designed to allow for magnetic coupling between the two ferromagnetic layers 811 and 812 so that the multilayered free ferromagnetic stack 810 as a whole behaves like a free ferromagnetic layer which can be switched via a driving current across the MTJ and can be stabilized by a magnetic biasing layer 201 in the MTJ. This multilayered free ferromagnetic stack 810 can also be used to implement a free layer in other structures, such as the examples shown in FIGS. 1A, 4, 5, 6A, 6B, 7A and 7B. When used as a magnetic memory device, the first ferromagnetic layer 811 in contact with the barrier layer 130 primarily functions as the recording layer to store a recorded bit in either of the two directions.

The multilayered free ferromagnetic stack 810 can be designed to provide a number of advantages that may be difficult to achieve in a free layer that has only a single material. For example, the non-magnetic spacer layer 813 can function as a diffusion barrier layer to block undesired diffusion and crystalline orientation propagation during post deposition annealing.

As another example, a ferromagnetic material can exhibit the magnetostriction effect, i.e., the dimension and shape of the material change with the magnetic field. The magnetostriction effect is undesirable in part because any variation in shape or dimension from one MTJ cell to another in an MTJ array can lead to variations in behaviors of MTJ cells and thus degrade the device performance. The use of two ferromagnetic layers 811 and 812 in the sandwich multilayer structure for the multilayered free ferromagnetic stack 810 allows the sandwich multilayer structure to be designed to reduce the net magnetostriction effect of the sandwich multilayer structure for the multilayered free ferromagnetic stack 810. For example, the materials for the two ferromagnetic layers 811 and 812 can be selected to have a near zero magnetostriction effect. When this is not possible or difficult to achieve, the materials for the two ferromagnetic layers 811 and 812 can be selected to exhibit magnetostriction in opposite directions so that their magnetostriction effects negate each other in the sandwich multilayer structure for the multilayered free ferromagnetic stack 810 to achieve a zero or near zero net magnetostriction.

Hence, the sandwich multilayer structure for the multilayered free ferromagnetic stack 810 provides engineering flexibility in selecting materials for the layers 811, 812 and 813 and designing the overall properties of the structure that can be difficult to accommodate in a single-material free layer design.

The relative magnetization directions of two ferromagnetic layers 811 and 812 in the multilayered free ferromagnetic stack 810 can be implemented in two configurations. The first configuration is an anti-parallel configuration where the magnetization directions of two ferromagnetic layers 811 and 812 are set to be opposite to each other due to anti-ferromagnetic coupling between the two layers 811 and 812. The anti-ferromagnetic coupling between the two layers 811 and 812 maintains this anti-parallel configuration so that the magnetization directions of the two layers 811 and 812 are switched together. The second configuration is a parallel configuration where the magnetization directions of two ferromagnetic layers 811 and 812 are set to be parallel to each other due to the ferromagnetic coupling between the two layers 811 and 812. The ferromagnetic coupling between the two layers 811 and 812 maintains this parallel configuration so that the magnetization directions of the two layers 811 and 812 are switched together. The two configurations are different. Under the spin torque transfer switching, the anti-parallel configuration tends to have a higher threshold switching current than that in the parallel configuration. Hence, the parallel configuration can be advantageously implemented in some devices to achieve a low switching current and thus reducing the power consumption.

For two given ferromagnetic layers 811 and 812, the non-magnetic spacer layer 813 between the layers 811 and 812 can be configured to set the multilayered free ferromagnetic stack 810 in either of the two configurations. For a given electrical conductive and non-magnetic material as the spacer layer 813, the thickness of the spacer layer 813 can be controlled to achieve either the ferromagnetic coupling or anti-ferromagnetic coupling. When the spacer layer 813 is made of ruthenium, for example, an anti-ferromagnetic coupling is achieved when the thickness of the Ru layer 813 is approximately 3, or 7, or 18, or 31 angstroms. When the thickness of the Ru layer 813 is between 3 and 7, or 7 and 18, or 18 and 31 angstroms, the ferromagnetic coupling is achieved between the layers 811 and 812.

In implementations, a ferromagnetic material suitable for two ferromagnetic layers 811 and 812 can include, for example, Co, an alloy comprising Co, Fe, an alloy comprising Fe, Ni, or an alloy comprising Ni. As a specific example, a ferromagnetic material can include CoFe, CoFeB, CoFeTa, NiFe, CoPt, CoPd, FePt, CO₂Mn(Al, Si) or CO₂(Cr, Fe) (Al, Si). The non-magnetic spacer layer 813 can be, for example, Ru, Re or Cu. Due to the presence of the magnetic biasing layer 201 that magnetically stabilizes the multilayered free ferromagnetic stack 810, each MTJ cell, which includes the magnetic biasing layer 201, the multilayered free ferromagnetic stack 810, the insulator barrier layer 130 and the pinned ferromagnetic layer 111, can be patterned to have a low aspect ratio between dimensions along two orthogonal directions parallel to each layer. This low aspect ratio can be set to around 1 to increase the cell density and to be compatible with CMOS fabrication processing.

As a specific example for the sandwich multilayer structure of the multilayered free ferromagnetic stack 810, the first ferromagnetic layer 811 in contact with the pinned layer 130 can be CoFeX, where X includes at least one of B, P, Si, Nb, Zr, Hf, Ta, and Ti. The second ferromagnetic layer 812 in contact with the magnetic biasing layer 201 can be a layer of a soft magnetic material such as NiFe or NiFeCo alloy, which has near zero magnetostriction. The composition of the CoFeX material for the first ferromagnetic layer 811 and the composition of the material for the second ferromagnetic layer 812 can be selected to have opposite magnetostriction effects so that the total magnetostriction of the free layer stack 810 can be close to zero. One way to achieve the desired material compositions is to control the relative percentages of the elements (i.e., Co, Fe and X) in CoFeX for the first ferromagnetic layer 811 and the relative percentages of the elements in NiFe or NiFeCo for the second ferromagnetic layer 812.

The non-magnetic layer 813 can be made sufficiently thin so that two ferromagnetic layers 811 and 812 are magnetically coupled. This magnetic coupling between the two ferromagnetic layers can be in any of or a combination of a number of magnetic coupling mechanisms. One example of such magnetic coupling mechanisms is ferromagnetic coupling through the RKKY (Ruderman-Kittel-Kasuya-Yosida) interlayer coupling based on nuclear magnetic moments or localized inner d shell electron spins in a metal by means of an interaction through the conduction electrons. Another example is the “orange peel” magnetostatic coupling induced by the interface roughness of the non-magnetic spacer layer. A third example is magnetic coupling via spatial discontinuities such as pin holes formed in the non-magnetic spacer layer 813. The material for the non-magnetic spacer layer 813 can be any one of Ru, Re, Cu, Ta, or a non-magnetic alloy containing any of these materials. The thickness of the nonmagnetic layer is ranged from zero to tens of angstroms. As a specific example, a thin non-magnetic layer of 4 angstroms to 10 angstroms can be used.

The magnetic biasing layer 201 can be made of a single AFM layer such as PtMn and can have a thickness thinner than the critical thickness associated with a strong pinning field. Alternatively, the magnetic biasing layer 201 can have a multiple layer structure with a nonmagnetic layer insertion. As described above, the magnetic biasing layer 201 enhances the in-plane anisotropy or coercivity of the adjacent multilayered free ferromagnetic stack 810 and also provides a weak pining field to the adjacent multilayered free ferromagnetic stack 810. The magnetic orientation of the magnetic biasing layer 201 can be set so that the weak pinning field in the adjacent multilayered free ferromagnetic stack 810 is at a direction different from the pinned magnetization direction of the pinned layer 111 below the tunnel barrier layer 130.

FIG. 9 illustrates the offset in the magnetization directions between the weakly pinned multilayered free ferromagnetic stack 810 and the pinned layer 111. When magnetization direction of the multilayered free ferromagnetic stack 810 is set at an angle θ different from pinned layer direction, the starting torque value generated from the spin torque transfer current is large, and has much less variation compared to the case when the angles are left to variation and distribution caused by thermal random agitation. Under this offset design, the average STT switching current is reduced and its distribution at a high switching speed (e.g., with a switching time on the order of ns) can be minimized. In implementations, the off-set angle θ can be set to a value between zero to 180 degrees, e.g., between zero to 135 degrees. This off-set angle can be achieved, for example, through a second anneal of MTJ films (or patterned devices) in a lower magnetic field and at a lower temperature, compared to the first annealing for the pinned layer (PL) setting (PL/PtMn).

While the specification of this application contains many specifics, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination.

Only a few examples are described. One of ordinary skill in the art can readily recognize that variations, modifications and enhancements to the described examples may be made. 

1. A magnetic memory device, comprising: a substrate; a magnetic cell configured to have a memory bit with two states, the magnetic cell formed on the substrate and comprising: a multilayered free ferromagnetic stack having a net magnetization direction that is changeable between a first direction and a second substantially opposite direction such that the first and second substantially opposite directions correspond to the two states of the memory bit, the multilayered free ferromagnetic stack comprising first and second ferromagnetic layers and a non-magnetic spacer between the first and second ferromagnetic layers; a magnetic biasing layer in contact with and magnetically coupled to the multilayered free ferromagnetic stack to increase coercivity of the multilayered free ferromagnetic stack, the magnetic biasing layer configured such that the magnetization direction of the multilayered free ferromagnetic stack is changeable between the first direction and the second substantially opposite direction in response to a write electric current passing through the magnetic cell along a direction perpendicular to the first and second ferromagnetic layers to switch between the two states of the memory bit; a fixed ferromagnetic layer having a fixed magnetization direction, and an insulator barrier layer formed between the multilayered free ferromagnetic stack and fixed ferromagnetic layer to effectuate tunneling of electrons between the multilayered free ferromagnetic stack and fixed ferromagnetic layer under a bias voltage applied between the multilayered free ferromagnetic stack and fixed ferromagnetic layer and across the insulator barrier layer, wherein the multilayered free ferromagnetic stack is located between the magnetic biasing layer and the insulator barrier layer; and a circuit coupled to the magnetic cell, the circuit configured to perform a read operation and a write operation, wherein the circuit is configured such that in the read operation the circuit applies a read electric current passing perpendicularly through the magnetic cell that is insufficient to switch the magnetization direction of the multilayered free ferromagnetic stack, and wherein the circuit is configured such that in the write operation the circuit applies the write electric current greater than the read electric current to the magnetic cell sufficient to switch the magnetization direction of the multilayered free ferromagnetic stack based on spin transfer effect caused by the write electric current passing perpendicularly through the magnetic cell, and wherein the magnetic biasing layer is configured such that a switched magnetization direction of the multilayered free ferromagnetic stack is maintained after removal of the write electric current.
 2. The device as in claim 1, wherein the non-magnetic spacer between the first and second ferromagnetic layers in the free ferromagnetic layer comprises Ru, Re, Ta or Cu, or an alloy that comprises Ru, Re, Ta or Cu.
 3. The device as in claim 2, wherein the non-magnetic spacer has a thickness between 4 and 10 angstroms.
 4. The device as in claim 1, wherein a ferromagnetic material for at least one of the first and second ferromagnetic layers in the multilayered free ferromagnetic stack comprises Co, an alloy comprising Co, Fe, an alloy comprising Fe, Ni, or an alloy comprising Ni.
 5. The device as in claim 4, wherein a ferromagnetic material for at least one of the first and second ferromagnetic layers in the multilayered free ferromagnetic stack comprises CoFe, CoFeB, CoFeTa, NiFe, CoPt, CoPd, FePt, Co₂Mn(Al, Si) or Co₂(Cr,Fe)(Al, Si).
 6. The device as in claim 1, wherein: the first ferromagnetic layer is in contact with the insulator barrier layer and comprises CoFeX, where X comprises at least one of B, P, Si, Nb, Zr, Hf, Ta and Ti, and the second ferromagnetic layer is in contact with the magnetic biasing layer and comprises a soft magnetic material.
 7. The device as in claim 6, wherein the second ferromagnetic layer comprises NiFe or NiFeCo.
 8. The device as in claim 6, wherein the first and second ferromagnetic layers have opposite magnetostriction effects.
 9. The device as in claim 8, wherein the multilayered free ferromagnetic stack has a net magnetostriction near zero.
 10. The device as in claim 1, wherein the multilayered free ferromagnetic stack is weakly pinned by the magnetic biasing layer to have a magnetization direction in the first direction which is different from the fixed magnetization direction of the fixed ferromagnetic layer.
 11. The device as in claim 10, wherein the magnetization direction in the first direction of the multilayered free ferromagnetic stack is offset by an angle between zero and 135 degrees from the fixed magnetization direction of the fixed ferromagnetic layer.
 12. The device as in claim 1, wherein the magnetic cell has an aspect ratio of less than 1.77 between dimensions along two orthogonal directions parallel to the substrate.
 13. The device as in claim 12, wherein the magnetic cell has an aspect ratio of around 1 between dimensions along two orthogonal directions parallel to the substrate.
 14. The device as in claim 1, further comprising: an antiferromagnetic (AFM) layer formed in contact with the fixed ferromagnetic layer to pin the magnetization of the fixed ferromagnetic layer along the fixed magnetization direction.
 15. The device as in claim 1, wherein the magnetic biasing layer comprises an antiferromagnetic (AFM) layer which is magnetically coupled to the multilayered free ferromagnetic stack to increase the coercivity of the multilayered free ferromagnetic stack while allowing switching of the magnetization direction of the multilayered free ferromagnetic stack.
 16. The device as in claim 15, wherein the magnetic biasing layer comprises IrMn or PtMn.
 17. The device as in claim 1, wherein the magnetic biasing layer comprises at least one of PtPdMn, NiMn, FeMn, FeMnRh, CrMn, FeNiMn, CoMn, RhMn, CrMnPt, CrMnCu, CrMnRh, CrMnPd, CrMnIr, CrMnNi, CrMnCo, CrMnTi and CrAl.
 18. The device as in claim 1, wherein the magnetic biasing layer comprises a metallic antiferromagnetic material.
 19. The device as in claim 1, wherein the magnetic biasing layer comprises an oxide antiferromagnetic material.
 20. The device as in claim 19, wherein the magnetic biasing layer comprises at least one of Ni(Fe)O, Fe(Co)O, Co(Fe)O, NiFe(Co)O and CrO.
 21. The device as in claim 1, wherein the magnetic biasing layer comprises FeS.
 22. The device as in claim 1, wherein the magnetic biasing layer comprises a ferrimagnetic material.
 23. The device as in claim 22, wherein the magnetic biasing layer comprises at least one of TbCo, DyCo, TbFe, TbFeCo, CoFeO, FeO, MnFeo and ZnFeO.
 24. The device as in claim 1, wherein the magnetic biasing layer comprises two antiferromagnetic layers.
 25. The device as in claim 1, wherein the magnetic biasing layer comprises: first and second antiferromagnetic layers; and a spacer layer between the first and second antiferromagnetic layers.
 26. The device as in claim 1, wherein the magnetic biasing layer comprises: an antiferromagnetic layer; and a spacer layer in contact with the antiferromagnetic layer.
 27. The device as in claim 26, wherein the magnetic biasing layer further comprises: a second spacer layer in contact with the antiferromagnetic layer; and a second antiferromagnetic layer in contact with the second spacer layer.
 28. The device as in claim 1, wherein the first and second ferromagnetic layers are anti-ferromagnetically coupled through the non-magnetic spacer to have opposite magnetization directions.
 29. The device as in claim 1, wherein the first and second ferromagnetic layers are ferromagnetically coupled through the non-magnetic spacer to have parallel magnetization directions.
 30. The device as in claim 1, further comprising: a plurality of magnetic cells formed on the substrate and arranged in rows and columns to form an array, and wherein the circuit comprises: circuit elements coupled to the array of the magnetic cells to make row selection and column selection in the array of the magnetic cells; and a write-current source to supply the write electric current for writing data in a magnetic cell to switch the magnetization direction of the multilayered free ferromagnetic stack based on spin transfer effect caused by the write electric current flowing perpendicularly through the magnetic cell.
 31. A magnetic memory device, comprising: a substrate; and an array of a plurality of magnetic cells formed on the substrate and arranged in rows and columns, wherein each magnetic cell is configured to have a memory bit with two states and comprises: a multilayered free ferromagnetic stack having a net magnetization direction that is changeable between a first direction and a second substantially opposite direction such that the first and second substantially opposite directions correspond to the two states of the memory bit, the multilayered free ferromagnetic stack comprising first and second ferromagnetic layers and a non-magnetic spacer between the first and second ferromagnetic layers, a magnetic biasing layer in contact with and magnetically coupled to the multilayered free ferromagnetic stack to increase coercivity of the multilayered free ferromagnetic stack, the magnetic biasing layer configured such that the magnetization direction of the multilayered free ferromagnetic stack is changeable between the first direction and the second substantially opposite direction in response to a write electric current passing through the magnetic cell along a direction perpendicular to the first and second ferromagnetic layers to switch between the two states of the memory bit, a fixed ferromagnetic layer having a fixed magnetization direction, and an insulator barrier layer formed between the multilayered free ferromagnetic stack and fixed ferromagnetic layer to effectuate tunneling of electrons between the multilayered free ferromagnetic stack and fixed ferromagnetic layer under a bias voltage applied between the multilayered free ferromagnetic stack and fixed ferromagnetic layer and across the insulator barrier layer, wherein the multilayered free ferromagnetic stack is located between the magnetic biasing layer and the insulator barrier layer, wherein the magnetic cell is configured to allow the magnetization direction of the multilayered free ferromagnetic stack to be switched by spin transfer effect caused by the write electric current passing perpendicularly through the magnetic cell, and wherein the magnetic biasing layer is configured such that a switched magnetization direction of the multilayered free ferromagnetic stack is maintained after removal of the write electric current. 